Fix UX consistency

[alexevanczuk]

Summary (please read before review)

This fixes the issue from #2

TLDR: danger-packwerk can only leave GitHub comments where there is a visible diff in GitHub (this is a GitHub restriction). Since we don't want to leave top-level PR comments, this makes it so certain new violation additions can never show up in Github. Namely, if we add a new violation against a constant with existing violations, the existing constant may not be part of the diff.

I have two implementations, as separate commits, to fix this in this PR:

1. I toggle between leaving the comment on the constant OR the file, with a preference for the constant if it's in the diff. I like this becuase I feel like the comment on the constant draws attention to that constant being the focus of attention. Although note the message will always contain information about the constant. I don't like this approach because its a lot more complex (leaving room for error and harder maintenance) and also exposes more in the public API (although I could probably address this with a different design).
2. I simply always leave the comment on the new file. This simplifies things and is also a more consistent experience for the user. It simplifies the solution a bit too.

Do you prefer solution (1) or solution (2)? Let me know!

Commits

• clean up common test setup
• clean up let blocks
• remove unnecessary context
• add test case for single constant addition
• add test for new reference against existing constant
• fix test by changing behavior
• add another test case
• switch to simpler implementation
• Revert "switch to simpler implementation"

[shageman]

Simplicity of implementation pushes me to solution #2

[alexevanczuk]

Simplicity of implementation pushes me to solution #2

Sounds good @shageman . I've dropped the revert of solution 2 and force pushed along with a commit to bump the version.